The liquid tank includes an aquarium or observation container, and particularly tanks for raising and breeding fishes and the like thereinside are common. Such tanks are usually composed of a transparent material. In an aquarium tank containing water 21 as shown in FIG. 9, for example, ambient light is reflected on the external and internal (outer and inner surfaces) of an observer-side transparent plate (the transparent wall 11), and this makes it difficult for the observer to clearly view the inside of the tank and also to photograph it clearly because strobe light is reflected. With a view to clearly observe aquarium or food fishes raised or bred in the tank from the outside, modifications and improvements have been made not only for the tank itself such as a viewing window and a transparent wall but for maintenance tools installed on the tank, including an air pump and a cleaning tool.
For example, as a way of more clearly viewing underwater objects, it is commonly known that an anti-reflection thin film is coated on the external surface of a glass or acrylic resin transparent wall constituting a tank, or alternatively, an anti-reflection film is attached to it, thereby preventing reflection of ambient light on the external surface of the wall (hereinafter, also referred to as “external surface-reflection”).
With regard to a technology for preventing reflection on an interface between a content of the tank and the internal surface (the interior of the tank) of the wall (hereinafter, also referred to as “internal surface-reflection”) in addition to the external surface-reflection, for example, Patent Document 1 discloses that at least an observer-side one of the transparent walls constituting the tank has an anti-reflection function on its both surface.
Examples of the liquid tank also include a tank equipped with a window (viewing window) used as a liquid volume meter and through which objects present in the liquid can be viewed. Also in such a tank, reflection on the external and internal surfaces of the viewing window occurs in the matter of the above-described tank, so that an observer has a difficulty in accurately measuring the volume of the liquid.
In the tank inside which fishes are raised and bred, there is required to minimize or completely prevent adhesion of algae to the wall surface, thereby maintaining the wall surface clean and keeping the fishes in captivity healthy. As a technology of achieving it, for example, Patent Document 2 discloses a film for lining a water tank, provided with a tacky layer having the transparency and water resistance on one surface of a transparent resin film, further a release sheet or a sheet of paper on the surface thereof and a peeling off means for peeling at least one end of the release sheet or paper.
Similarly in the liquid tanks, the clear under-liquid observation is difficult also in viewing devices for under-liquid observation, including goggles and aquascopes. In goggles, for example, reflection on a lens surface might occur.
As a technology of preventing such surface reflection, a “moth-eye structure”, a kind of nanostructure, is known in the field of optical materials. The moth-eye structure includes a great number of nanometer-sized conical protrusions formed on a transparent substrate surface, for example. As disclosed in Patent Documents 3 to 6, the moth-eye structure allows continuous change of the refractive index from air to the transparent substrate, so that incident light does not recognize the interface as an optical surface, and as a result, reflection of the light can be significantly decreased.    [Patent Document 1]    Japanese Kokai Publication No. 2003-319733    [Patent Document 1]    Japanese Kokai Publication No. Hei-09-322674    [Patent Document 1]    Japanese Kokai Publication No. 2003-43203    [Patent Document 1]    Japanese Kokai Publication No. 2005-156695    [Patent Document 1]    WO 2006/059686    [Patent Document 1]    Japanese Kokai Publication No. 2001-264520